1. Field of the Invention
The present invention relates to a line type thermal head applied to, for example, a thermal printer, which is comprised of a one-dimensional array of heat generation resistors, and to a thermal transfer recording apparatus equipped with such a thermal head.
2. Description of the Related Art
FIG. 6A shows one form of a conventional thermal head. A thermal head 1 comprises a plurality of heat generation resistors 51 of a parallelogrammic configuration formed on an insulating substrate 50, such as ceramics or alumina, and arranged at a predetermined interval in a linear array, a pair of lead electrodes 52, 53 formed on both ends of the resistor 51, and external terminals 54 and 55 connected to the lead electrodes 52 and 53. The opposite sides of the lead electrodes 52 and 53 are defined along the array of the resistors 51 and the lead electrode 52 is continuous to provide a common electrode.
In the thermal head as set out above, the size of a recording dot can be varied by varying an amount of energy to be applied to the resistor 51. This is because the resistors, constituting the thermal head, is parallelogrammic in configuration and allow an energy concentration to occur due to an energy distribution one-sided in the resistors 51. It is thus possible to better record a medium-tone image on a recording sheet.
The basic technique of the thermal head will be explained below in more detail.
When a voltage is applied across the lead electrodes 52 and 53 in the thermal head as set out above, a flow field in the heat generation resistors 51 is as shown in FIG. 7. In FIG. 7, black points represent points of measurement where the sense of their line shows a sense of current at the point of measurement and the length of the line shows a magnitude of current at the point of measurement.
FIG. 7 shows a view for explaining how the current distribution in the resistors is as shown n FIG. 7. Let it now be assumed that the values of the resistors 51 do not vary by their heat generation. The heat generation resistors are each formed of, for example, a thin film, though having somewhat a very small thickness, and can be regarded as being a two-dimensional plane in which case the thickness of the resistor is disregarded. Based on the aforementioned assumption, the current distribution in the resistors 51 becomes a stationary electric current field. Since no magnetic density B (Bx, By) varies in the stationary electric current field, the following equation holds with the use of the "Maxwell equation" ##EQU1##
Further, a current density i (ix, iy) establishes the following equation with the use of the Law of Conservation of Electric Charge EQU divi=0 (2)
with a conductivity .delta. and electric field E (Ex, Ey), EQU i=.delta.E (3)
with the use of the Ohm's law.
Substituting Equation (3) into Equation (2) yields EQU divE=0 (4)
From Equations (1) and (2) the following equation holds in the presence of a scalar function V: EQU E=-grad V (5),
noting that Vin Equation (5) stands for a potential.
Substituting Equation (5) into (4) gives a Laplace's differential equation below: ##EQU2##
Also, the energy density en can be expressed as follows: EQU en=iE=.delta.E.sup.2 ( 7)
Solving Equation (6), as well as Equation (5) for the electric field E, gives a distribution of heat energy from Equation (7).
Equation (6) is numerically analyzed using a "boundary element method". Here the boundary element method comprises dividing a boundary of a closed system into a plurality of elements, as shown in FIG. 8, and finding a solution for every element with the use of predetermined boundary conditions. By so doing, it is possible to find the inner state of the closed system.
In this way, it is possible to obtain a flow field as shown in FIG. 7.
As appreciated from Equation (7), electric current shows a greater value as it goes toward the middle of the heat generation resistor 51. Further, a quantity of heat generated at any given point in the resistor 51 is expressed by a product of a squared quantity of electric current at that location and a resistive value of the resistor 51, that is, is proportional to a square of the electric current. Thus the quantity of heat generation is great at the middle of the resistor 51.
On the other hand, more quantity than a predetermined quantity of generation heat is required for dot recording. If a smaller voltage is applied to the resistor 51, dots are recorded over a heat generation range as indicated by 61a in FIG. 7. With an increasing application voltage, dots are recorded over a heat generation range as indicated by 61b, 61c in FIG. 7.
A substantially heat generation area can be varied as indicated, for example, by 61a, 61b and 61c by applying a varying amount of energy to the resistor 51. It is thus possible to modulate a dot size.
On the other hand, since an electric current distribution in the resistor 51 varies depending upon the size of the resistor 51, the resistor may assume a specific shape for the most suitable half-tone printing, a shape which generates a concentrated heat to an extent exceeding a certain level. Here, for a parallelogram having typical values, a ratio g between a length La of one side 51a and a length Lb of a side 51b intersecting with the side La and an angle .theta. (here an acute angle) made between these sides 51a and 51b as shown in FIG. 9 are given below, as an optimal shape,
(1) the ratio of (b/a).ltoreq.1 PA1 (2) the angle .theta..ltoreq.45.degree. PA1 (1) a ratio [1], [1.5], [2] at an angle .theta. of 30.degree. PA1 (2) a ratio [1], [1.5], [2] at an angle .theta. of 45.degree. PA1 (3) a ratio [1], [1.5], [2] at an angle .theta. of 60.degree. PA1 (4) a ratio [1], [1.5], [2] at an angle .theta. of 75.degree. PA1 (1) the ratio .ltoreq.1 PA1 (2) the angle .theta..ltoreq.45.degree. PA1 (1) The thermal head is displaced in accordance with color to be recorded. PA1 (2) The plurality of heat generation elements are so arranged that they correspond to one pixel. One of the plurality of heat generation elements is selectively used in accordance with color to be recorded. PA1 (3) The plurality of thermal heads are arranged in a direction of conveyance of the recording sheet such that they are displaced a predetermined amount in a direction orthogonal to that in which the recording sheet is conveyed. One of the plurality of thermal heads is selectively employed in accordance with color to be recorded. PA1 (4) The recording timing or conveyance of the recording sheet is varied by a predetermined extent in accordance with color to be recorded. PA1 (5) The plurality of thermal heads for recording elongated dots with their major axes differently oriented in their individual directions are arranged in the direction of conveyance of the recording sheet and one of the plurality of thermal heads is selectively used in accordance with color to be recorded. PA1 (6) The thermal head is so arranged that one of the plurality of heat generation elements for recording elongated dots on the recording sheet with their major axes differently oriented in their individual directions is selectively used in accordance with color to be recorded.
The above matter has already been proposed by the present applicant under Japanese Patent Application 1-195686 (1989).
The optimal shape of the resistor 51 so set will be briefly explained below. Here a thermal head as applied to a G3 facsimile will be explained below by way of example.
In the G3 facsimile equipment, since an image resolution in a horizontal scanning direction (a direction of the resistor array) is defined as 8 [dots/mm], the width, that is, the length La of the resistor 51 is given by EQU La.ltoreq.125 .mu.m
with a resistor-to-resistor gap given by 25 .mu.m and the resistor 51 set as large as possible, EQU La=100 .mu.m
Here consideration is paid to combinations of the angle and ratio g as given below.
For a plurality (here 12) of types of resistor shapes, a current distribution may be considered by the aforementioned method with the outline of the resistor as a boundary as shown in FIG. 7, provided that La=100 .mu.m, a potential on the lead electrode 53=24 V, and a potential on the lead electrode 52=0 V.
Further, an electric field E in the horizontal scanning direction and diagonal direction (see FIG. 9) is evaluated and an energy density en calculated based on the electric field with the use of Equation (7) is divided by the conductivity .delta., that is, en/.delta.. From this it follows that the smaller the angle .theta. and ratio g the greater the center concentration of the electric current.
Noting the ratio g it is found that, for g=[2], the energy distribution is substantially uniform and that there is almost no energy concentration. It is also found that a smaller energy concentration is developed at the ratio g=[1.5] and that a marked energy concentration occurs at the ratio [1.5]. Further, noting the angle .theta. at the ratio g=[1], an energy concentration is pronounced at the angle .theta. of below 45.degree..
From the above it may be inferred that the following equation is established for the optimal shape of the resistor 51.
If a thermal head is to be constructed for application to the G3 facsimile equipment, the resistor 51 is made at a height of about 70 .mu.m or below in views of its width=100 .mu.m so as to obtain an optimal shape. Further, the resistor has an optimal size if the image resolution in a vertical scanning direction is above 15.4 [lines/mm], provided that the height is below 70 .mu.m.
In a currently available ordinary facsimile equipment (G3), a resolution in a vertical scanning direction, such as 7.7 [lines/mm]relative to 8 [dots/mm], is lower than the aforementioned resolution 15.4 [lines/mm]. It has, therefore, been difficult to construct such an ordinary low-resolution thermal head.
FIGS. 6A and 6B show a conventional thermal head and dots recorded by the thermal head, respectively, the array of dots being shown in the vertical scanning direction, that is, in the feed direction of a recording sheet. As shown in FIG. 6B, the recording dots become, for example, elliptic in shape due to a concentrated energy in the resistors of the thermal head as set out above. The major axes of the elliptic recording dots are made oblique relative to the vertical scanning direction.
In the conventional thermal printer, a "separation" direction as will be set forth below is the same as the vertical scanning direction and dots are recorded in the elliptic state with their major axes oblique in the vertical scanning direction as shown in FIG. 6B. If any spacing is left between the respective adjacent recorded dots, however, ink transfer tends to be unstable at almost all edge portions of the dots due to the greater edge portion of respective dots present and the greater edge portion of the respective dots made oblique in the "separation" direction. This gives rise to a very poor quality image.
As set out above, the conventional thermal recording apparatus and hence the thermal head records dots as elliptic ones in which case their major axes are made oblique in the vertical scanning direction with a spacing left between the adjacent recorded dots. This causes a very unstable ink transfer upon the separation of an ink film from a recording paper and a greatly degraded image.
In recent times, a growing demand is made for a colored document to be transmitted as data by a facsimile equipment and a recording apparatus (color printer) for color recording has vigorously been developed in this field of art. In the conventional color printer, dots are recorded as colored dots of predetermined shape, though somewhat different for their shape and size depending upon the recording conditions. With attention paid to one pixel in a recording image, recording has to be made with a color dot almost completely superimposed on a previous color dot in the same position. If an almost complete superimposition is achieved between these color dots in the same position, a resultant color dot is sharply distinguishable from another color dot incompletely superimposed in a common dot position. As a result, a moire (interference fringe) is produced depending upon the types of image patterns. In the color recording in particular, an image quality is degraded due to the occurrence of the moire.